The Tritanium Uncemented Prosthesis Versus the Cemented Triathlon Prosthesis

There have been many studies published that have shown a reduction in bone density around knee replacements. Exact reasons for this are not fully understood but one theory is that the stresses transmitted to the bone around a new knee replacement do not adequately load the surrounding bone due to the stress shielding effects of the cement. Bone requires itself to be loaded by weight-bearing pressure in order to maintain its density and structural integrity.

A loss of bone density around the knee has potentially significant consequences for patients. The ability of the bone to support the knee replacement may be compromised, potentially leading to subsidence and failure of the joint replacement. Patients are known to be more prone to falls within 1 year of having a TKR in comparison to age matched controls who have not undergone knee replacements. The incidence of falls following knee replacement was shown to be 38% in one study after the first year post-operatively, compared to 24% in non-TKR subjects. A reduction in bone density following TKR also renders patients more prone to aseptic loosening and fracture around the knee replacement with potentially disastrous consequences.

The majority of research seems to demonstrate a loss of BMD around the knee at around 12 months following TKR surgery, with the most significant BMD decrease occurring between the period of 5-12 weeks post-surgery, occurring primarily at the periprosthetic region. Up to 68% of patients have been reported as showing BMD loss after TKR at the distal anterior femur within the first 12 months post-operatively. The possible fracture risk associated with this has been investigated. Women aged ≥ 70 years who have undergone TKR surgery are 1.6 times more likely to have a fracture than younger patients and 2.3 times more likely to suffer a fracture than men.

Bone density is measured by use of Dual X-ray Absorbitometry scans (DEXA). However, DEXA bone density measurement alone provides an incomplete picture of bone strength. DEXA measurement is derived from the bone mineral content (BMC) divided by bone area, but does not account for the distribution of the trabeculae and the structural integrity of the microarchitecture. This has led to the development of a tool called the trabecular bone score (TBS) which is able to differentiate between microarchitectures that exhibit the same density.

Advancement in technology has led to re-designs in uncemented knee replacements. These newer implants are able to withstand torsional stresses that previously would have concerned surgeons who feared implant movement in the early post-operative phase. The development of biologic coatings on to TKR components also means that bony on-growth and thus stability of the knee implant is achieved much more quickly than was previously. Surgeons have become keener to use such uncemented devices and recent research has shown comparable results in terms of patient satisfaction, Oxford Knee Scores and the need for revision surgery between cemented and uncemented devices.